The invention relates to methods of manufacturing radiation detectors and radiation imaging devices, radiation detectors and imaging devices manufactured by these methods and the use of such imaging devices.
A typical method of manufacturing a radiation detector for an imaging device comprises applying a layer of a metal such as aluminium to both of the main surfaces of a planar semiconductor substrate, applying a layer of pbotoresistive material to cover the semiconductor material, exposing the photoresistive material on the surface of the planar substrate with an appropriate mask pattern, removing the photoresistive material to expose a pattern of the metal to be removed, etching away the metal to be removed and then removing the remaining photoresistive material to leave a pattern of contacts on one surface of the substrate and a metallised layer on the other surface of the substrate. The contacts on the first surface of the substrate then define an arrangement of radiation detector cells.
For optical wavelengths and charged radiation (beta-rays), silicon has typically been used for the semiconductor material for the substrate. A method of the type described above has been used to good effect with this material.
In recent years, cadmium zinc telluride (CdZnTe) has increasingly been proposed as a more suitable semiconductor material for use in X-ray, gamma-ray and to a lesser extent beta-ray, radiation imaging. CdZnTe is good at absorbing X-rays and gamma-rays giving better than 90% efficiency for 100 keV X-rays and gamma-rays with a 2 mm thick detector. The leakage or dark current of these detectors can be controlled and values of the order of 10 nA/cm2 or less at 100 Volts bias are achievable.
A small number of companies worldwide currently produce these detectors commercially in a variety of sizes and thicknesses. Usually one or both sides of the planar detectors are contacted with a continuous metal layer such as gold (Au) or platinum (Pt). As mentioned above, such detector substrates then need to be processed to produce a detector having a pattern of contacts (e.g. pixel pads) on one surface, with the opposite surface remaining uniformly metallized, in order that the detector may be position sensitive, that is in order that the detector is able to produce a detector output indicating the position at which radiation impacts the detector. A readout chip then can be xe2x80x98flip-chipxe2x80x99 joined to the patterned side of the CdZnTe detector (e.g., by bump bonding using balls of indium or conductive polymer material, gluing using one-way conductive materials or other conductive adhesive layer techniques) so that the position dependent electrical signals which result from incidence and absorption in the detector cells of X-rays or gamma-rays can be processed. The readout chip could be of the pulse counting type with very fast integration and processing time (typical a few microseconds or at most a few milliseconds). Alternatively, it may be one of type described in the Applicant""s International Patent Application PCT/EP 95/02056 which provides for charge accumulation for individual detector cells. With art imaging device as described in PCT/EP 95/02056, integration times can be several milliseconds, or tens or hundreds of milliseconds. As the signal integration or standby/readout period increases it becomes more critical that the gold or platinum contacts on the CdZnTe surface are electrically separated to a high degree to avoid signals from neighbouring contacts (pixel pads) leaking and causing the contrast resolution to degrade.
It has been found that the traditional method of forming the contacts on the detector surface, particularly when CdZnTe is used as the semiconductor material, does not provide as high an electrical separation of the contacts as would be desired to make optimum use of the advantages which are to be derived from the imaging devices as described in the International Application PCT/EP 95/02056, which is incorporated herein by reference.
In accordance with one aspect of the invention, there is provided a method of manufacturing a radiation detector having conductive contacts on a semiconductor substrate at positions for defining radiation detector cells, wherein said method includes steps of.:
a) forming one or more layers of material on a surface of said substrate with openings to said substrate surface at said contact positions;
a(i) forming a layer of passivation material on said substrate surface;
a(ii) forming a layer of photoresistive material on said passivation layer;
a(iii) selectively exposing said photoresistive material and removing said photoresistive material from areas corresponding to said contact positions to expose said passivation layers;
a(iv) removing said passivation material from said areas exposed in step a(iii) corresponding to said contact positions to expose said substrate surfaces;
a(v) removing remaining photoresistive material of said photoresistive material layer;
a(vi) forming a further layer of photoresistive material on said exposed passivation layer and exposed substrate surfaces; and
a(vii) selectively exposing said further layer of photoresistive material and removing said further photoresistive material in a pattern corresponding to said contact positions;
b) forming a layer of conductive material over said layer(s) of material and said openings; and
c) removing conductive material overlying said layer(s) of material to separate individual contacts, including:
c(i) removing said further layer of photoresist material.
The use of an insulating layer of passivation material means that after manufacture of the detector, the passivation material remains between the contacts protecting the semiconductor surface from environmental damage in use and further enhancing the electrical separation of the contacts.
The present inventors have found that the surface resistivity of cadmium-based substrates, for example a CdZnTe semiconductor substrate is degraded when the substrate is exposed to metal etchants suitable for removing gold and/or platinum. As a result of this, the electrical separation of the individual contacts which result from the conventional method of forming such contacts is not as good as would be expected from the properties of that material before treatment. By using a lift-off method in accordance with the invention, metal etchants need not be used, thus avoiding the damage which would result if the metal etchants came into contact with the semiconductor surface. By removing a first pbotoresistive layer and subsequently applying a further photoresistive layer improved adherence to the passivation material may be obtained. Moreover, the mechanical integrity of the further layer of photoresistive material is greater than the first layer, and consequently lift-off of subsequently formed layers of material, e.g conductive material, may be achieved more reliably and has been found to provide a higher production yield for devices manufactured using this process.
Furthermore, the further layer of photoresistive material allows for the exposure of areas larger than the contact positions such that conductive material may be applied over portions of passivation material adjacent to the contact positions. Further photoresistive material may even be applied to areas or regions away from, yet operatively related to, contact positions. This is particularly advantageous and is intended for use in manufacturing high energy (1 KeV) radiation imaging devices since it allows more complex conductive material patterns and/or second conductive material layers to be formed on the passivation material. For example, for off-setting a charge collection contact of a detector cell relative to a corresponding contact of a readout substrate cell. Additionally, conductive material may extend from the contact positions over adjacent portions of the passivation material, thereby providing good mechanical contact, and reducing the possibility of gaps being formed between the conductive material and passivation material.
In a preferred embodiment the further photoresistive material is removed from an area corresponding generally to said contact positions. Preferably, the further photoresistive material is removed from an area greater than said contact positions to expose adjacent portions of said passivation material. In this way, gaps between conductive material and passivation material can be avoided.
Optionally, the further photoresistive material is removed from areas of said passivation material to expose said areas in a desired pattern for forming conductive tracks.
To protect the other main surface and the sides (edges) of the semiconductor substrate, photoresistive material can additionally be applied to all exposed surfaces prior to step a(iv).
In a preferred method in accordance with the invention, prior to step a(iv), a photoresistive material is additionally applied to all exposed surfaces.
The invention finds particular, but not exclusive use with substrates formed of cadmium telluride (CdTe) or cadmium zinc telluride (CdZnTe). It will be appreciated that the method of the invention can be used with other substrate materials as well.
Typically, the conductive layer is a metal or metal alloy or cadmium sulfide.
Preferably, the metal layer for forming the contacts is applied by a method such as sputtering, evaporation or electrolytic deposition, preferably by sputtering.
Preferably, the metal layer for forming the contacts comprises gold (Au), although other metals, for example platinum (Pt) or indium (In), could also be used.
Preferably, the passivation layer is formed of aluminium nitride (ALN).
Each-metal contact can define a respective pixel cell of an array of pixel cells, or one of a plurality of strips arranged parallel to each other, depending on the application of the detector.
With a method according to the invention, the metal contacts can be of the order of 10 xcexcm across with a spacing of the order of 5 xcexcm.
The invention also provides a method of manufacturing a radiation detector, comprising a semiconductor substrate with a plurality of conductive contacts for respective radiation detector cells on a first surface thereof and layer of conductive material on a surface of the substrate opposite to the first surface, the conductive contacts being formed on the first surface by a method as described above. The layer of conductive material can be formed on the opposite surface of the substrate prior to step (a) of the method described above.
Suitably, the conductive material or contacts are of metal or metal alloys.
The invention further provides a method of manufacturing a radiation imaging device, comprising manufacturing a radiation detector as defined above, and individually connecting individual contacts for respective detector cells to corresponding circuits on a readout chip, for example by a flip-chip technique.
In accordance with another aspect of the invention, there is provided a radiation detector comprising a semiconductor substrate for detecting radiation with a plurality of conductive contacts for respective radiation detector cells on a first surface thereof and with a layer of conductive material on a second surface of said substrate opposite to said first surface, wherein the overall width of a said conductive contact is larger than the width of said contact adjacent said substrate.
In a preferred embodiment of the invention the semiconductor substrate is made of cadmium zinc telluride (CdZnTe), although other semiconductor substrate materials, for example cadmium telluride (CdTe), could be used. Preferably also passivation material is provided between individual contacts. Aluminium nitride has been found to be particularly effective as a passivation material for CdZnTe because it can be applied at low temperature, CdZnTe being temperature sensitive.
The metal contacts can define an array of pixel cells, or a plurality of strips arranged parallel to each other, depending on the field of use of the detector.
Pixel contacts formed on detector substrate are preferably substantially circular and are arranged in a plurality of rows, more preferably with alternate rows preferably being offset from adjacent rows.
The metal contacts are of the order of 10 xcexcm across wit a spacing of the order of 5xcexcm.
In detectors in accordance with the invention, the resistivity between metal contacts should be in excess of 1 Gxcexa9/square, preferably in excess of 10 Gxcexa9/square, more preferably in excess of 100 Gxcexa9/square and even more preferably in excess of 1000 Gxcexa9/square (1 Txcexa9/square).
The invention also provides a radiation imaging device comprising a radiation detector as defined above and a readout chip having a circuit for accumulating charge from successive radiation hits, individual contacts for respective detector cells being connected by a flip-chip technique to respective circuits for accumulating charge.
A radiation imaging device in accordance with the invention finds particular application for X-ray, gamma-ray and beta-ray imaging.
Thus an embodiment of the invention can provide a method for manufacturing, for example, detectors having a CdZnTe substrate with one side uniformly metallised with gold and the other side patterned with gold structures in a manner that does not adversely affect the surface characteristics of the CdZnTe substrate between the gold structures. Thus, a method can be provided for creating gold structures on one side of a CdZnTe detector, the method achieving interstructure resistivity of the order of Gxcexa9/square or tens or hundreds of Gxcexa9/square. The gold structures may be patterned to provide readout tracks, for example.
The use of an electrically insulating passivation layer between contacts further enables the area between metal contacts to be protected, thus giving the detector stable performance over time and avoiding effects such as oxidation which increase the surface leakage current and decrease the inter-contact resistivity. Aluminium nitride (AlN) passivation has been found to be particularly effective when applied between gold contacts to protect the surface and enhance the electrical separation of the gold contacts. The passivation layer of aluminium nitride can be implemented at relatively low temperatures typically less than 100xc2x0 C. By contrast, silicon oxide (SiO2), which is typically used as a passivant for silicon (Si) semiconductors, needs temperatures in excess of 200xc2x0 C. After exposure to these temperatures, CdZnTe would be unusable.